YOU SAID:

I don't understand why he's giving this lecture.

INTO JAPANESE

彼がなぜこの講義をしているのか理解できません。

BACK INTO ENGLISH

I can not understand why he is giving this lecture.

INTO JAPANESE

なぜ彼がこの講義をしているのか理解できません。

BACK INTO ENGLISH

I can not understand why he is giving this lecture.